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To harness the power of functions in C++, it's essential first to understand how to declare them. A function declaration provides a compiler with the function's name, return type, and parameters, but not the body of the function itself. This enables other parts of your program to call the function even before its definition.

In the context of our exercise, the function `findSmallest` would be declared in the following way:

• double findSmallest(double num1, double num2, double num3);

This signifies that `findSmallest` is expected to return a double value, which aligns with our use of double-precision floating-point numbers, and it will take three double parameters that it will compare. Placing this declaration at the beginning of your program or in a separate header file is a best practice that improves readability and organization.

Double-precision floating-point numbers are an essential data type in C++ when high precision in large or small numbers is necessary. Unlike single-precision floating point (float), a double in C++ offers about 15 to 17 decimal places of precision, because it allocates 64 bits to store a number, compared to 32 bits used for a float.

In our program to find the smallest number, using double ensures that the function can handle numbers with a significant degree of accuracy, which can be particularly important for scientific calculations or when precision is vital. It's important to be consistent in using double for all related variables to avoid unnecessary type conversions and the potential loss of precision.

Conditional statements allow you to execute different pieces of code based on certain conditions. In C++, `if-else` statements are one of the most commonly used conditional statements. They enable your program to branch its execution path depending on whether a condition evaluates to true or false.

In the function `findSmallest`, an `if-else` structure decides which of the three passed-in numbers is the smallest. Here鈥檚 a simplified logic using pseudocode:

• If num1 is less than both num2 and num3, return num1.
• Else, if num2 is less than num3, return num2.
• Else, return num3.

Each condition is checked in order until one is found to be true, and the related piece of code executes. It is essential to consider all possible scenarios to ensure accurate results, especially when dealing with multiple conditions as in our example.